1. Field of the Invention
The present invention relates to digital photography. In particular, the described embodiments relate to the extraction of color information from an image.
2. Related Art
Portable digital cameras and video cameras have become very common in all aspects of everyday life. With the explosion of personal computer technology over the past decade, it is possible to transmit data which is representative of images captured by a digital still camera or video camera in a format recognizable by a personal computer. The image data received at the personal computer may then be manipulated, edited or reproduced on media using a color printer. Digital cameras typically focus a scene or an object through optics onto an imaging array which captures the focused image. Instead of capturing the image on photographic film as in conventional photography, however, a digital camera typically captures the image on a semiconductor imaging sensor which is suited for the capture and reconstruction of color images. Signals representative of the captured image are then transmitted from the semiconductor imaging sensor to a memory for further processing. The processed image may then be transmitted to a personal computer or some other device capable of reproducing the image on a particular medium.
The typical semiconductor imaging sensor absorbs photon energy at various locations leading to the creation of carrier pairs (i.e., electron and hole pairs). Circuitry formed in the semiconductor imaging array stores the charge resulting from the carriers during an exposure period for the semiconductor imaging array. Following the exposure period, the charge stored in the circuitry is read out and processed to reconstruct the image.
The charge is typically collected in the semiconductor substrate by applying an electric field to separate holes and electrons in the carriers. In a charge couple device (CCD) arrangement, a metal oxide semiconductor (MOS) capacitor is formed and an electric field is induced by applying a voltage to a gate of the capacitor. In a CMOS imaging array, a photodiode is formed in the semiconductor having a junction with a built-in field. The photodiode can be reverse biased to further enhance the field.
Once the semiconductor imaging array absorbs photon energy, resulting in the creation of an electron-hole pair, it is not possible to determine the wavelength or the color of light associated with the photon energy. To detect color information, typical imaging sensors control the color of light that is permitted to be absorbed into the substrate. This is achieved in some systems by employing a prism to decompose a full color image into its component colors and using an individual imaging device to collect the image for each of the component colors. This requires precise alignment of the imaging devices and therefore tends to be very costly.
Household video cameras typically use microfilter technology to control the color of light that is allowed to reach any given pixel location. For such a video camera with a semiconductor sensor (or sensing element) array, each detector in the semiconductor imaging array, therefore, is used to detect the intensity of a particular color of light at a particular location in the imaging array. Such filters are typically directly deposited onto each of the light sensing elements formed in the imaging array. The filter color pattern deposited on a given sensing element in the imaging array controls the color of light detected by the particular element.
While camera optics produce an image of a scene which has full color depth at each point in the image, only one color is collected at any particular location. A typical imaging sensor uses red, green and blue as primary colors, including red, green and blue transmissive filters distributed uniformly throughout the imaging array. The intensity of the photon energy collected at each of the pixel locations is typically represented by eight bits at each pixel location. Since much of the light incident at a pixel location is filtered out, color information is lost. Using red, blue and green as the primary colors, the original image would have 24 bits of color data at each location. A color filter pattern using red, blue and green filters deposited at different pixel locations in a specific pattern, known as the Bayer pattern, is discussed in detail in U.S. Pat. No. 3,971,065.
Much color information is lost at any particular pixel location using Bayer pattern. Accordingly, there is a need to provide a cost effective system for extracting additional color information from a semiconductor imaging sensor.
An object of an embodiment of the present invention is to provide an imaging sensor which is capable of providing high resolution images.
It is another object of an embodiment of the present invention to provide a method and system for capturing additional color information from images in digital photography.
It is another object of an embodiment of the present invention to provide an improved CMOS based imaging sensor.
It is another object of an embodiment of the present invention to provide a low cost digital imaging sensor which can capture color information to more fully exploit color printing technology.
It is yet another object of an embodiment of the present invention to provide an imaging sensor which directly measures color information at pixel locations in a xe2x80x9cwhitexe2x80x9d spectral region.
Briefly, embodiments of the present invention are directed to an imaging array comprising a first set of light-sensitive elements and a second set of light-sensitive elements. Each of the first set of light-sensitive elements have a sensitivity to energy in one of a plurality of spectral regions which are substantially distinct from each other. Each of the second set of light-sensitive elements have a sensitivity to energy in a spectral region which includes substantially all of the spectral regions of the first set of light-sensitive elements. The second set of light-sensitive elements are preferably distributed among the first set of light-sensitive elements substantially uniformly throughout the array.
By having the second set of light-sensitive elements, the imaging array is capable of capturing wideband spectral information from an image, in addition to narrower band information captured at the first set of light-sensitive elements. The wideband spectral information provides measurements of the intensity of photo-exposure by xe2x80x9cwhitexe2x80x9d light at locations of light-sensitive elements. The wideband spectral information may then be employed to provide a more accurate reproduction of images at, for example, a color printer.